\section{Wi-Fi: 802.11 Wireless LANs}
Since the wireless network technology was invented in 1991 \cite{kurose}, 
a lot of technologies and standards for wireless networks have been developed, for 
example the HiperLan and HiperLan II \cite{hiperlan}, 
HomeRF \& SWAP \cite{homerf} and Bluetooth \cite{bluetooth}, but the winner of all these standards is  
the \textbf{IEEE 802.11 wireless LAN} standard \cite{802.11}, also known as \textbf{Wi-Fi}.
The terms 802.11 and Wi-Fi are often used interchangeably, but they are
focusing on different aspects. 

The IEEE 802.11 is a set of standards defined by the IEEE Working Group for
Wireless Local Area Networks.  
The standards make specifications in different areas of wireless LANs such
as
MAC- and physical layer, power management, authentication and encryption
etc. 

Wi-Fi is a brand owned by the Wi-Fi Alliance
\cite{wifi_alliance}.
As the 802.11 standards were finished and vendors brought their products to
the market, there were problems of interoperability due to the complexity
of the standards. To solve the problem, the Wi-Fi Alliance, a non-profit
organisation, was founded in 1999.
Today it has more than 300 members from more than 20 countries \cite{wifi_about}.
It's main task field lies in increasing the compatibility and interoperability
of the products based on the IEEE 802.11 standards. 
Products with the Wi-Fi logo 
guarantee interoperability with other
Wi-Fi certified products. 

Therefore, the term \emph{Wi-Fi} addresses the interoperability and is 
used mostly in
this thesis, the term \emph{IEEE 802.11} is  used when referring to the technology underlying
Wi-Fi.
\subsection{802.11 Architecture and Network Components}
In the following subsections, we present the
802.11 architecture and its network components.
The definitions are based on 802.11 MAC and physical
layer specifications \cite{802.11_1999}.
\begin{itemize}
\item In 802.11, a \textbf{station} is a computer using the 802.11
technology. It can be a fixed computer, a portable or a mobile one. 
\item If at
least two stations are communicating with each other using 802.11, they are
building a \textbf{basic service set (BSS)}. A BSS can be independent and
spontaneous, i.e., there is no coordinator of the set, 
then we say that the stations
are operating in \emph{Ad-hoc Mode}. If there is a coordinator in the BSS
which is responsible for medium access, authentication, encryption, IP
address assignment, traffic forward etc., then we call the coordinator  \textbf{Base
Station} or \textbf{access point} (AP), or we say that the stations are operating in
\emph{Infrastructure Mode}.
\item Two or more BSS are interconnected with a \textbf{Distribution System}
(DS) and
can be integrated in the LAN. In this way, an \textbf{Extended Service Set}
(ESS) is created.
Figure \ref{img:ap} illustrates the most import components of an ESS with 
two basic service sets 
operating in  
the infrastructure mode.
\end{itemize}
\begin{figure}[htb]
 \centering
 \includegraphics[width = 13cm]{pics/ap}
 \caption{Components in an ESS of IEEE 802.11 LAN architecture}
 \label{img:ap}
\end{figure}

An access point normally runs a \emph{Dynamic Host Configuration Protocol}
(DHCP) \cite{rfc2131} server. A client can request network parameters like IP address,
subnet mask, default gateway and DNS etc. from the DHCP server. The
DHCP server must ensure that an IP address assigned to a client is unique.
Using DHCP facilitates the network configuration and administration.

In this thesis, we only discuss the \emph{infrastructure mode}, because the
mobile client needs to get an IP address for Wi-Fi sharing and the access
points take part in the authentication process and filter traffic.
In a typical home network, the AP and the router are often packaged in a
same box that connects the BSS into the Internet. In our thesis, we call
such a box \emph{access router} or \emph{middlebox}.

\subsection{Wi-Fi Mobility Category}\label{sec:mobility_def}
Being wireless, mobility is an instinct nature of Wi-Fi. 
At least in the radio propagation, a user can move
with
his mobile device in a range of up to 100 meters. But from the view of
network layer, the IP address remains the same and it is not mobile at all.
We call this kind of mobility \emph{static} (from the view of
the network layer).

A mobile user can roam from one access point to another, get disassociated
with 
the old one and associated with the new one with a new IP address
assigned. In
this way, the old session is closed and a new session starts.
This kind of roaming will not be regarded
as mobility, because no handover is required.

If a mobile user moves from one access point to another which are both in
the same subnetwork, and gets the same IP address from the new AP, the
ongoing connections can remain. However, this kind of mobility is only
possible at places where all APs belong to the same ESS, i.e., the
Distribution System is the Intranet of an institution. According to
\cite{paul_tan}, we call this kind of 
mobility \emph{Micro Mobility}.

A mobile user can move from one access point to another which are
interconnected over the Internet (i.e. from one ESS to another), and get 
a new IP address. According to \cite{paul_tan},
if he manages
to maintain an ongoing connection even if in this circumstance, we call it
\emph{Macro Mobility}. 
\subsection{Wi-Fi Sharing Communities}
%\subsubsection{Was is Wi-Fi sharing community}
In Wi-Fi sharing communities, members share their bandwidth with each
other. 
The idea is quite simple, if a user lets other
members get the Internet access over his access point, he will be able to
use others' access points for the Internet as services in retain.

There are both technical and cultural reasons for the emerging of Wi-Fi sharing
communities. Technically, the standardization of the 802.11 technology and
especially the foundation of the Wi-Fi Alliance make the interoperability
of products from different vendors possible. The broadband subscriber
normally have  contract with their ISP at fixed costs. Through the large
coverage area of the 802.11 b/g products \cite{802.11_coverage}, it is technically possible to
share an Internet connection with neighbours. Culturally, these grass root
movements act in the sense of freedom and share, which can be considered as
an outgrowth of the free software communities. 

There are many  Wi-Fi sharing networks. FreeNetworks.org
\cite{freenetworks} has a list of the most important wireless networks in
the world. Among those, the largest community for Wi-Fi sharing is FON,
which we introduce in the following subsections.

FON was founded by the Argentinean entrepreneur Martin Varsavsky in
November 2005. In February 2006, FON gained venture capital of over 20
million US dollars from Skype,
Google and other companies \cite{nyt}. As of October 2007, FON has already
more than 500.000 registered members \footnote{Derived from the member
number of a test registration on the 10th of October 2007}, among those
more than 300.000 wireless routers have been installed \cite{fon_number}.

FON adopted the grass root style community to a business model. In FON,
there are three kinds of participants: 
\begin{itemize}
\item \emph{Aliens}\\
People who are not members of the community but want to use the FON
hotspots for the Internet. They must buy a ticket for their Internet access which
is normally much cheaper than using commercial hotspots.
\item \emph{Linus}\\
People who provide their
bandwidth for free to others and then get free Internet access from
other community APs 
everywhere in the world. 
\item \emph{Bill}\\
Community members who  want to earn money from their access points. They share half of the
revenue from their access points with the company FON. If they roam to
other places, they must pay for using of community hotspots.
\end{itemize}
FON's business model is based on selling tickets to Aliens.
Furthermore, FON also cooperates ISPs to expand the community. Recently,
Martin Varsavsky announced the cooperation between FON and British Telecom
to found the BT FON community \cite{bt_fon}. Additionally, FON also promotes
the construction of municipal Wi-Fi by providing Wi-Fi routers to city
citizens for free \cite{fon_city}.
%\subsubsection{Warum sind solece Communites entstanden?}
%\subsubsection{Welche Wi-Fi sharing communites gibt es?}
%\subsubsection{Die groesste is FON}
% Hintergründe zur Gründung
% Busieness model, Linus, BILL, Alien
% Risikokapital von Google und Skype
% Die neueste Entwicklung von FON
